\documentclass{article}
\usepackage{amssymb,amsmath,amscd,concmath}
\pagestyle{empty}
\begin{document}
\begin{equation*}
a = \frac{(b - a)(b + a)}{(b - a)} = \frac{(b - a)}{(b - a)}(b + a) = (b + a)
\end{equation*}
\end{document}
